High speed infrared photodetectors are becoming increasingly important for optical communication signal processing, infrared imaging and measurement systems. Photodetectors can be used for the detection of optical communication signals, such as those used in the telecommunication industry. A standard wavelength used in the telecommunication industry is 1.3 microns. Pure silicon photodetectors are limited in that they can only detect wavelengths up to about 1 micron. The addition of germanium to a silicon film used to fabricate a photodetector can increase the wavelength that can be detected (i.e., photodetector responsivity). For example, using a silicon-germanium film which incorporates more than 50 percent germanium can increase the photodetector responsivity to 1.3 microns or more. However, adding germanium to the silicon film often results in a strain to the silicon crystal lattice that may causes defects, such as threading dislocations, to be formed in the silicon-germanium film. The proliferation of such defects can result in an increase in the defect concentration, or defect density, in the silicon germanium film, which can impair the performance of a silicon germanium photodetector device.
Accordingly, there is a need for improved methods of forming a silicon germanium film and structures formed thereby which incorporate greater than 50 percent germanium concentration while exhibiting low defect concentrations. The present invention provides such methods and structures.